Sod webworm is likely to become a problem in nonirrigated turf during this dry weather. Larvae are attacked and controlled in moist turf with a naturally occurring microsporidian--a small, funguslike organism. When turf stays dry, more sod webworm larvae survive to cause heavy damage.
Larvae live in silk-lined tunnels in the thatch, emerging at night to feed on grass blades. Damage appears initially as irregular, brownish turf areas. Continued feeding results in large, brown areas. On close inspection, the brown areas are due to the thatch showing because the grass blades have been eaten off at the crown. It is common to see small balls of green webworm feces at the base of the plants.
Positive identification of sod webworms involves using a disclosing solution. A teaspoon of 6% pyrethrin or tablespoon of dishwashing detergent is added to a gallon of water and then applied evenly over a square foot of turf. Within a minute or so, sod webworm larvae, black cutworm larvae, earthworms, rove beetles, and other insects come to the surface. Sod webworm larvae are elongate caterpillars up to 1 inch long, with dark brown spots. The background ranges from greenish to gray to tan. Two or more sod webworms per square foot are enough to cause damage.
Presence of large numbers of insectivorous birds such as starlings, cowbirds, and robins feeding on the turf for several days in succession also indicates sod webworms. Although the birds are eating the larvae, they are unlikely to eat enough to prevent webworm damage. Also, the birds create 1/2-inch-diameter holes through the thatch. Heavy webworm infestations may result in severe bird injury as well.
Finally, one can predict sod webworm infestation by observing the adult moths. The slender, 1-inch, tan moths sit on the turf and fly up when disturbed. They typically fly less than 10 feet above the turf in a jerky, up-and-down motion. After flying only about 30 feet, they drop back to the turf, where they can be observed more closely. At rest, the wings fit tightly around the body, making the insect look more tubelike than other moths. If there are many moths and the turf stays dry, application about 2 weeks later should provide excellent control of hatching larvae.
Bifenthrin (Talstar), carbaryl (Sevin), diazinon, halofenozide (Mach 2), spinosad (Conserve), and trichlorfon (Dylox) should be effective in controlling larvae. Insecticidal nematodes such as Steinernema carpocapsae should also be effective.